The present disclosure relates generally to systems and methods for magnetic resonance imaging (MRI), in particular, to systems and methods for designing parallel transmission (pTx) radiofrequency (RF) pulses for focused tissue treatment.
Outside surgical interventions, treatments for tissue abnormalities, such as malignant tumors, typically include techniques aimed at directly killing cells using high energy beams of ionizing particles, such as photons, electrons, protons and other ions. Although a variety of conformal approaches have been developed, aiming to minimize damage to healthy tissues due to radiation exposure, such systems and methods maintain considerable risks and side-effects, especially for deep-seated targets. For example, in the case of photons, deposition of ionizing energy is peaked closer to the surface, in dependence of beam energy, resulting in significant energy deposition along the path of the beam. By contrast, protons, and other charged particles, exhibit strongly non-linear absorption profiles, or Bragg peaks, in tissue, whereby strong energy deposition occurs at depth with little absorption occurring closer to the tissue surface. Nevertheless, such absorption profiles include tails that are still very significant, and result in death or injury to healthy tissues outside the target.
In addition, ionizing radiation presents further issues on account of difficulties associated with dose monitoring. For example, computer simulations are commonly used to predict the dose distribution, but these are subject to programming errors, errors in the segmented models of the subject, and incorrect registration between the subject and model. Also, monitoring of response to treatment may be done using CT, PET and MR imaging after the treatment, but there are no easy ways to verify the dose during a treatment session. Furthermore, ionizing radiation only provides for lethal killing of cells, and so other treatments, such as hyperthermia, are not possible with this technique.
By contrast, focused ultrasound waves can achieve local “point” heating in portions of a subject anatomy, such as the brain or torso, and can therefore be used as alternative approaches for hyperthermia treatment. However, a significant drawback of such methods stem from the nature of propagation of ultrasound energy in the body. For example, in the case of non-invasive brain treatment, ultrasound waves must pass through the skull. Since the skull acts as a sound wave insulator, achieving significant heating inside the brain necessitates increasing the power of the ultrasound device. As a result, most of the additional energy is dissipated in the skull, as well as outer skin and fat layers of the head, and not in the brain. This may cause significant and undesirable heating of the skull, which may necessitate extraneous cooling systems methods. In fact, temperature increases in the skull may be many times larger than that of the target.
By contrast, RF-based hyperthermia treatment generally involves heating target cells by exposing them to an intense RF field. Unlike sound waves, however, radiofrequency (RF) waves are not stopped by a skull, and thus treatment with RF waves may not significantly affect a skull or any other non-target tissue. In some cases, it may be desirable to achieve a modest temperature rise that would not kill specific cells, such as tumor cells, but instead render them more responsive to specific biological agents. For example, when used in conjunction, chemotherapy treatment, RF-based hyperthermia may allow for reduction in chemotherapy dose, thus reducing side effects, while increasing efficiency. Therefore, unlike for the case of ionizing particles, the intensity of RF-based hyperthermia treatment can be adjusted continuously to induce smaller temperature increases that warm the tumor cells, or larger temperature rises that would destroy them. As a result, the ability to continuously adjust the intensity of heating can be used to adapt the treatment to the patient-specific requirements of personalized medicine.
Previously, parallel transmission (pTx) RF pulse design algorithms have been proposed, in the context of magnetic resonance imaging applications, for achieving uniform excitation of spins across a field of view. Due to versatility on account of a large number of degrees of freedom, pTx RF pulse techniques have provided important approaches toward addressing RF and static field inhomogeneity issues, which are particularly problematic at high magnetic fields. Specifically, electromagnetic (EM) simulations have shown that the interference of electric fields created by multiple RF channels can deposit large amounts of energy, as measured by specific absorption rates (SARs), with multiple focal locations in the body. When not controlling local SAR explicitly in the pTx pulse design process (i.e. when controlling global SAR or pulse power), local SAR in pTx is often found to be 5 to 10 times greater than in single channel excitations. As such, it is generally the goal of previous RF pulse design algorithms to find excitation patterns that minimize specific absorption rate (SAR) requirements, in accordance with regulated limits, while optimizing RF pulse performance and adhering to hardware constraints.
It would therefore be desirable to provide systems and methods for designing pTx RF pulses for use in RF treatment applications that overcome the shortcomings of currently available methods.